1. Field of the Invention
The field of the invention is that of the transmission of all types of signals, analog and/or digital. More precisely, the invention concerns transmission systems with amplitude modulation requiring high performance characteristics with respect to the noise induced by the transmission channel.
The invention can be applied notably, but not exclusively, to the transmission of television signals, irrespectively of their nature and their encoding, on all types of media such as cabled networks, community antennas, radio-frequency broadcasting etc. The invention can also be applied to sound signals and to data signals, especially when a high quality of transmission is required.
2. Description of the Prior Art
The basic principle of amplitude modulation is the multiplication (in the sense of signal processing) of the signal to be transmitted, or primary signal, by a carrier signal, which is sinusoidal for example, having a frequency that is at least twice as high as the maximum frequency of the carrier signal. The resultant of this multiplication, or modulated signal, is received by the demodulator.
In the demodulation, the transmitted signal undergoes a second multiplication by the carrier signal. This is followed by a low-pass type of filtering enabling the useful signal to be recovered.
For this second multiplication, the recovery of the carrier signal at the demodulator can be done almost synchronously, by the filtering and restriction of the received signal. However, this method generates post-filtering distortions. To get rid of these defects, it is possible to carry out a synchronous demodulation, the carrier frequency being recovered by a phase control loop.
In the case of television signals, and more generally in the case of all signals for which it is necessary to know the polarity, the modulation rate does not exceed 100%. This means that the amplitude deflection of the primary signal does not exceed half the amplitude of the high frequency signal obtained after modulation. If not, it is no longer possible to automatically control a phase locking lock at reception to recover the carrier frequency. The result thereof is that the potential of this modulation is under-utilized in terms of noise. For, the greater the modulation rate, the better are the performance characteristics of the modulation.
In a known way, suppressed carrier amplitude modulation (SCAM) corresponds to the limit case of the modulation at a modulation rate of over 100%, wherein the maximum amplitude of the primary signal is equal to the amplitude of the signal obtained after modulation.
The modulated signal then presents phase jumps of 0.degree. to 180.degree. directly related to the zero crossings of the primary signal. The fact that there are as many crossings through 0.degree. as there are through 180.degree. leads to the disappearance of the carrier frequency in the frequency domain. The synchronous demodulation of the signal can therefore no longer be done in a standard way. A particular phase control loop, known as the Costa's loop, is then used. This phase control loop obtains the products of the received signal by the carrier signal reconstituted in phase and in quadrature. The products obtained are applied to the input of the phase comparator of a standard phase control loop.
The periodicity of the Costa's control loop comparator is the periodicity .pi., while it is 2.pi. in the case of a standard demodulator. This leads to the impossibility of determining the polarity of a received signal. The non-recognition of the polarity of the signal has few drawbacks in the case of sound signals and, more generally, when the signal has no low frequencies, but makes it impossible to interpret certain signals, notably television signals.
In short, while the SCAM has a gain (of about 6 dB) in terms of performance characteristics with regard to noise and to interference units as compared with the simple amplitude modulation, and while it requires only little additional power, the loss of the polarity of the signal limits its applications.
Furthermore, there is another known technique of amplitude modulation, namely vestigial sideband amplitude modulation (VSBAM).
Amplitude modulation, whether of the standard type or of the suppressed carrier type, has the further drawback of causing a duplication of the useful frequency spectrum, hence a doubling of the spectral space factor. For a useful spectrum ranging from 0 to f.sub.c, and for a carrier frequency f.sub.p, the spectrum transmitted will range between the frequencies f.sub.p -f.sub.c and f.sub.p +f.sub.c. This spectrum includes two sidebands, on either side of f.sub.c, containing the same information.
Vestigial sideband amplitude modulation enables this spectral space factor to be reduced by partially eliminating one of the two sidebands, in such a way that the accurate restoration of the signal remains possible. This can be obtained by Nyquist slope filtering. The lower sideband is then partially eliminated, for example. In this case, it is an infradyne transmission, the modulation frequency f.sub.p being lower than the frequencies of the spectrum transmitted.
VSBAM has the advantage of reducing the spectral space factor by eliminating the redundancies. It is widely used for the transmission of television signals. However, its performance characteristics with respect to noise are limited in the same way as for the standard amplitude modulation as regards the performance characteristics of the SCAM.
It is an aim of the invention, notably, to overcome these drawbacks of the known amplitude modulation techniques.
More precisely, it is an aim of the invention to provide a system of amplitude modulation having characteristics with respect to noise and interference units identical to those of the suppressed carrier amplitude modulation (SCAM) while at the same time preserving the polarity of the transmitted signal.
It is notably an aim of the invention to provide a system such as this, enabling the transmission of signals of any type, digital and/or analog, and especially television signals for which the preservation of polarity is necessary. The invention may be used, for example, for the transmission of signals of the MAC (Multiplexed Analog Components) type.
It is a complementary aim of the invention, in a particular embodiment, to provide a modulation system such as this, with a low space factor, similar to that of the vestigial sideband amplitude modulation (VSBAM).
It is another aim of the invention to provide a modulation system such as this enabling the transmission of information complementary to the useful signal.
It is also an aim of the invention to provide receivers that provide for the reception of signals transmitted in simple vestigial sideband or suppressed carrier amplitude modulation, as well as signals transmitted according to the amplitude modulation technique of the invention.
A complementary aim of the invention is to use the principle of transmission of the invention to enable swift and certain locking into the phase control loop of such receivers.